Optical sensors have been proposed or implemented in medical devices for monitoring changes in blood or tissue oxygen concentration. When short-term, relative changes in oxygen concentration are monitored, calibration of the optical sensor is not necessarily required. For example, two-wavelength optical sensors can be used to detect relative changes in blood oxygen saturation over short intervals of time, e.g. less than one minute, without requiring calibration using oxygenated blood. However, determining absolute StO2 is desirable in many monitoring applications because absolute measurements allow changes in oxygen that occur over longer time intervals to be measured and compared and provides a better indication of the actual patient's oxygenation status at a given time.
Four wavelength optical sensors have been implemented in external monitoring devices, which allow absolute StO2 to be measured. Tightly controlled manufacturing specifications with narrow tolerances enable calibration of the external optical sensors to be performed by calibrating one device and applying the calibration results to all devices. The calibration procedure typically requires calibration using blood flowing in a loop and oxygenated at different saturation levels to obtain a calibration curve.
Maintaining stringent manufacturing specifications to enable a single calibration to be applicable to all devices, however, can add to the cost of manufacturing and the frequency of device rejection. Widening the tolerances of manufacturing specifications can reduce manufacturing costs with the trade-off that a single calibration curve may no longer be applicable to all sensing devices. Calibration of each individual device using a blood flow loop, however, would also add undue manufacturing cost and time burden. A need exists, therefore, for an optical sensing device and associated methods for monitoring absolute StO2 that does not require calibration of all individual sensing devices using a blood loop but still enables manufacturing without highly restrictive tolerances or complex components.